()是指能对人造成伤亡或对物造成突发性损害的因素。
A.危险因素
B.危害因素
C.有害因素
D.致病因素
参考答案：A
—___________ he have to wear his school uniform every day?
—No, he ______________.
A．Must; mustn't
B．Can; can't
C．Does; doesn't have to
D．Do; don't have
锅炉运行中，为什么要经常进行吹灰、排污？
